How long have you been doing this grow? Have you seen any patterns in terms of ph? I know that one of my buckets usually goes from 5.6 to about 6.1 over 12 hours, and my other bucket goes from 5.6 to about 5.9. I also know that every plastic tablespoon of my own ph down mix results in about a .10 drop in each bucket.
So if my meter stopped working, I would probably use put 4-5 spoonfuls of the mix in the one bucket every 12 hours, and about 2-3 in the other bucket. That should be pretty safe for a few days. And get on Amazon and order a new meter ASAP.